1. Field of the Invention
The present invention relates to an image processing apparatus that performs screen processing, and also relates to an image processing method including the screen processing.
2. Description of the Related Art
The method for forming an area gradation image is conventionally known and used to realize a tone reproduction of an image on a recording medium. The area gradation is characterized by changing the ratio of coloring material adhesion regions to express a gradation image. An amplitude modulation (AM) screen and a frequency modulation (FM) screen are representative ones.
The AM screen can express a gradation image by modulating the size of each coloring material adhesion region (i.e., the size of a so-called halftone dot). The factors defining the AM screen include the shape of each halftone dot, the direction along which halftone dots are arranged (i.e., angle of halftone dots), and the density of periodically arranged halftone dots (i.e., the number of lines).
On the other hand, the FM screen can be used to express a gradation pattern based on a change in the density of dots. To this end, isolated minute dots having a constant size are arranged in a pseudo random manner. If the FM screen is employed, it is required to record minute dots stably. Therefore, in a case where an image forming apparatus cannot form minute dots stably, it is useful to employ the AM screen to obtain a stable output.
A moiré pattern includes an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes. If the AM screen is employed to perform printing, original moiré may occur. The original moiré is a periodic pattern that may occur when a high-frequency component of an input image interferes with periodically arranged halftone dots. The high-frequency component of the input image can be visually recognized when folded into the low-frequency region. The following two methods are conventionally known as a practical method capable of reducing the original moiré.
The method discussed in Japanese Patent Application Laid-Open No. 08-051536 includes performing filter processing on an input image and removing a frequency component that may cause moiré from the input image. The method discussed in Japanese Patent Application Laid-Open No. 2007-129558 employs the FM screen including isolated minute dots arranged in a pseudo random manner, instead of using the AM screen, if the original moiré occurs when the AM screen is used.
However, the frequency component that may cause moiré is a high-frequency component that is adjacent to a screen frequency. Therefore, if the method discussed in Japanese Patent Application Laid-Open No. 08-051536 is employed, an input image may be blurred out.
Further, if the method using the FM screen as discussed in Japanese Patent Application Laid-Open No. 2007-129558 is employed by a recording apparatus that cannot form dots stably, a formed image may have noticeable roughness.